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McConnell’s Challenge

By

Jason L. Riley

Dec. 8, 2014 1:05 p.m. ET

The good news for Republicans is that Rep. Bill Cassidy’s victory over Louisiana Sen. Mary Landrieu in Saturday’s Senate run-off election gives the party a four-seat cushion in the Upper Chamber for the final two years of the Obama presidency. The bad news for Republicans is that the political map in the 2016 cycle could make that advantage fleeting.

Republicans will be defending 24 seats in two years, while Democrats will be defending 10. Making matters trickier, the GOP will be trying to hold seven seats in states that...